All APIs are different. At its core, an API provides direct access deep into a web service (lower case - a service that is provided on the web) and turns it into a Web Service (upper case) that people can use as a building block. What makes it an API is the infrastructure that sits in front of it, attracts developers to use it, secures it from misuse and provides the metrics and management needed to turn an internal web service into a Web Service managed through an effective distribution channel, and providing strategic and/or financial benefit.

While each API is different, the infrastructure I have described is consistent across virtually all of them, so it is neither economical nor effective to reinvent the wheel for each API someone wants to release. It is similar to the concept of an adserver - all websites have different content and fucntionality, but the concept of selecting and serving an ad, tracking it, and targeting it is pretty consistent across sites; as a result, there are many sites that use a handful of adserver providers.

In addition to allowing companies to focus on their core business without having to build peripheral, non-core services, using a third-party service who is focused on providing that service allows you to benefit from ongoing development and enhancement, and from features that would be prohibitively expensive to build for just a single provider.

As for an example? check out sites such as developer.trulia.com or developer.compete.com, our first two customers (we have many more, but I like to give props to our early adopters). In addition to documentation and community, they have developer key issuance, instant self-service developer provisioning, usage and rate throttling, and tracking. What you don't see, but our clients enjoy, is a dashboard where they can assign different access levels, rates or limits to each developer on a key-by-key basis, customize error messages and other API parameters, and see detailed reports of API usage on a developer-by-developer or overall basis. Building all of that takes time and money; we offer it as an instantly-deployable on-demand service with no up-front investment, and our customers seem to find it an excellent value.